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Furnace not turning on

Most no-heat calls turn out to be a setting, a switch or a filter. A few are not, and those are the ones to take seriously.

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The short version

Most likely cause
The thermostat, a switched-off furnace switch or a tripped breaker.
Safe to check
Thermostat mode and batteries, the furnace switch, the breaker and the filter.
Call a technician when
It starts and shuts off, clicks without lighting, or shows an error code.

Smell gas, or is a carbon monoxide alarm sounding? Leave the house first. Call your gas company or 911 from outside. Do not switch anything on or off on the way out.

Safe to check yourself

  • Set the thermostat to heat, above room temperature.
  • Replace the thermostat batteries if it has them.
  • Check the furnace power switch and the breaker.
  • Replace a dirty filter.

Stop and call

  • The furnace starts, then shuts off within minutes.
  • You hear clicking but no flame.
  • A light on the furnace is flashing a code.
  • There is soot, rust flakes or a burning smell.

Why a furnace will not start

  • ThermostatThe wrong mode, dead batteries or a loose wire will stop the call for heat before it reaches the furnace.
  • No powerA furnace has its own switch, often on the wall nearby, and its own breaker. Either can be off.
  • IgnitionA worn igniter or a dirty flame sensor lets the furnace try to light, then shut down as a safety measure.
  • A safety shutdownA badly clogged filter can overheat the furnace until its limit switch turns it off.

What a technician will do

  1. Read the furnace's error code.
  2. Test the thermostat signal and the power supply.
  3. Check the igniter, the flame sensor and the safety switches.
  4. Check the venting before putting the furnace back in service.

A furnace that shuts itself down is doing what it was built to do. Resetting it over and over without finding the reason defeats the safety device.

Related guides

Sources

  • US Department of Energy
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